The invention relates to a picture display device comprising a cathode ray tube having means for generating one or more electron beams, a display screen, and means for deflecting the electron beams across the display screen, the display screen being provided with index elements and the picture display device comprising receiving means for receiving signals generated by the index elements, and means for controlling the deflection and/or shape of the electron beam(s) in response to the signals.
Picture display devices of this type are known and are usually referred to as xe2x80x9cindexxe2x80x9d display devices or display devices of the xe2x80x9cindexxe2x80x9d type. As compared with the conventional picture display device, in which the cathode ray tube is provided with a color selection electrode (also referred to as shadow mask), such picture display devices have the advantage that, due to the absence of the shadow mask, they have a smaller weight, while the required energy is lower and the sensitivity to vibrations and temperature differences is reduced. This is offset by the fact that, due to the absence of a shadow mask, the sensitivity to disturbing effects of magnetic fields, including the earth""s magnetic field, is much greater and that much more stringent requirements are imposed on the accuracy with which the beams are deflected and generated.
The principle of the xe2x80x9cindexxe2x80x9d display device is based on the fact that, to obviate the above-mentioned drawbacks, the display screen is provided with index elements with which the position and/or the shape of the electron beam(s) can be controlled while they are being deflected across the display screen, which control data are used to correct the deflection and/or the shape of the electron beams. However, the problems caused during this control have hitherto been of such a nature that, in spite of the intrinsic advantages of the design, index display devices are not practical alternatives to the conventional designs.
It is an object of the invention to provide an improved picture display device of the type described in the opening paragraph.
To this end, a picture display device according to the invention is characterized in that the display screen comprises a first and a second pattern of index elements of conducting material, which elements are electrically separated, and the picture display device is provided with a first coupling between the first pattern and the receiving means and a second coupling between the second pattern and the receiving means.
In the known picture display devices of the index type, a pattern is used to generate index signals. The display device according to the invention comprises a first and a second pattern of index elements which are electrically separated. Consequently, two signals can be generated.
The invention is based on the following recognition.
Deviations from the xe2x80x9cidealxe2x80x9d position or shape of the electron beam(s) may have many causes, for example, influences of the earth""s magnetic field, low-frequency interference such as, for example, the mains (50 or 60 Hz), stray fields of neighboring electric apparatuses, interference with the video frequency, etc., etc. The low-frequency interferences generally cause interferences becoming manifest in relatively large offsets of the picture as a whole (for example, frame offsets or barrel-shaped distortions of the displayed picture). The high-frequency interferences cause many smaller deviations which become manifest in small deviations of the position of the electron beam on the screen. These deviations mainly cause color deviations because, locally, an electron beam partly impinges on a phosphor element of the xe2x80x9cwrongxe2x80x9d color. The display device according to the invention comprises a first and a second pattern of conducting index elements. During operation, a first and a second signal are generated thereby. These signals may be used for compensating interferences which are lower or higher than a given frequency. The given frequency is preferably the line frequency. The first coupling preferably comprises an electric filter for passing signals below a given frequency, and the second coupling preferably comprises an electric filter for passing electric signals above a given frequency. The frequency below which and above which electric signals are passed are preferably approximately equal. Crosstalk between the signals is thereby prevented.
The xe2x80x9clow-frequencyxe2x80x9d index patterns, couplings, receiving means, signals, etc. will hereinafter be referred to as xe2x80x9cDCxe2x80x9d index patterns, couplings, receiving means, signals, etc., and the xe2x80x9chigh-frequencyxe2x80x9d ones will be referred to as xe2x80x9cACxe2x80x9d.
Since a first and a second index pattern are provided, deviations can be better compensated, and with less energy. During operation, the first index pattern (the xe2x80x9cDCxe2x80x9d pattern) generates a signal which indicates frame distortions, frame offsets, frame rotations and other large-scale disturbances. This xe2x80x9cDCxe2x80x9d signal is applied to the means for receiving signals. The means for receiving signals derive signals therefrom for correcting the large-scale deviations. These signals are preferably applied to the deflection means. Consequently, large-scale deviations can be compensated very accurately. The first pattern preferably extends at least partly beyond a part of the display screen which is visible during operation, and the picture display device comprises means for causing the electron beam(s) to impinge on the part of the first pattern situated beyond the part of the display screen which is visible during operation.
The first pattern then extends at least partly across one or more blind edges (or part(s) of blind edges) around the visible part of the display screen. This provides the possibility of correcting the large-scale disturbances in the picture, even when the displayed picture is black or comprises only a small number of illuminated parts. A correction of high-frequency deviations can then be performed much better.
The first pattern preferably comprises a sub-pattern at least in each comer of the display screen. This provides the possibility of measuring differences between the corners, while frame offsets and rotations can be measured and therefore compensated. These sub-patterns preferably extend at least along a side (viewed from the corner) over a length of 20% of the length of the side. This provides the possibility of accurately measuring and thus correcting also barrel or pincushion distortions of the frame.
In addition to corner sub-patterns, a further preferred embodiment comprises sub-patterns along at least two sides of the display screen.
The first pattern preferably comprises means for determining the position of the electron beam in two mutually perpendicular (x and y) directions.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.